Enantioselective epoxidation of unfunctionalized olefins catalyzed by the Mn(salen) catalysts immobilized in the nanopores of mesoporous materials

Three Mn(salen) catalysts were immobilized in nanopores or on external surface of mesoporous materials via phenyl sulfonic groups with different linkage lengths. The influence of the axial linkage length and the nanopores or external surface of supports on the catalytic performance of the heterogeneous Mn(salen) catalysts was investigated in detail. The ee values increase with increasing linkage length for the Mn(salen) catalysts immobilized in nanopores but remain almost unchanged for those catalysts anchored on the external surface. The ee values obtained for the Mn(salen) catalysts immobilized in nanopore increase with decreasing nanopore size and they are generally higher than those of the same catalysts anchored on the external surface of support. Modification of the nanopore surface by methyl groups improves the reaction performance for the asymmetric epoxidation. The confinement effect originating from nanopores not only enhances the chiral recognition of the chiral catalyst, but also restricts the rotation of the intermediate in nanopores, enhancing the asymmetric induction and giving higher ee values than those obtained for the same catalysts anchored on the external surface.